Count how many long guns and pistols you have. Then double that number and go buy a safe that holds more than that number. Safes are like pole barns in the fact that most people run out of room in the first couple years of getting it. Also, look at things like mentioned above, ability to anchor to floor, fire ratings, quality of materials used, steel gauge, ability to configure and reconfigure interior, interior lighting if any. Champion and Liberty both come to mind to look at. Also, the below are good considerations brought up.

Gun safes are rated for unscoped single shot guns, if you put any thing with a scope or stacked barrell it places the butt of the gun out and then a 12 gun becomes an 8, double your current need minimum.